1. Field of the Invention
The present invention relates to methods of coating a metal substrates to improve tribological (friction and wear) thermal, chemical and electrical properties of the metal substrates and especially to methods of forming ceramic coatings on the metal substrates.
2. Description of the Prior Art
The practice of coating a metal substrate with a thin layer of a ceramic material has been used commercially for many years. One purpose of providing a ceramic coating of a metal is to improve the wear resistance of the metal from abrasion and another purpose is to protect the surface of the metal from thermal degradation, oxidation, or corrosion. In particular, research and development activity has been carried out throughout the world to provide superior surface properties to metals such as aluminum and aluminum alloys. Aluminum and aluminum alloys are extremely desirable metals for manufacturing machinery components and the like because they are relatively inexpensive and have relatively low densities. Aluminum and aluminum alloys, however, have the drawbacks of being relatively soft and do not resist wear and abrasion very well. In addition, aluminum is chemically active so that it tends to react with chemicals and even moisture in the air, thereby corroding.
A known method of improving the surface of a substrate of aluminum or an alloy of aluminum is to apply a ceramic coating to the substrate by spraying the ceramic coating onto the substrate. Typically, the process of "flame spraying" includes a wire-type flame sprayer. The protective coatings applied in this manner are limited to those materials which can be formed into a wire or rod.
Commercially available spray guns now permit the use of a wide variety of metals, alloys, ceramics, and cermets which can be ground into a relatively fine powder to coat the substrate. Before coating the substrate, the surface of the substrate is subjected to cleaning, under-cutting, and blasting to achieve good adhesion. The flame-spray gun utilizes combustion or a plasma flame to produce sufficient heat to melt the coating material. An electric arc or resistance here can also be used for generating the necessary heat. The carrier gas for the powder can be oxygen or even an inert gas such as nitrogen. In a plasma spray gun, the primary plasma gas is usually an inert gas such as nitrogen or argon.
Coatings of aluminum and alloys of aluminum are of particular interest as a thermal barrier in the rocket and jet engine field. Coatings for this intended use are designed to be somewhat porous in order to resist the high thermal shock encountered and to improve their effectiveness as thermal barriers. Therefore, these coatings are different from the fused coatings of low melting ceramics which are classed as vitreous enamels, and they must be applied over base materials if they are to be exposed to high temperatures and oxidizing atmospheres.
Ceramic coatings used in the prior art are generally inherently porous and ordinarily do not provide much oxidation or corrosion protection to the base material. Thus, undercoats made from oxidation-resistant materials, or alloys are used between the base material and the ceramic coating if the substrate material is not corrosion resistant.
Ceramic coatings containing aluminum oxide are used for wear protection because aluminum oxide is well known to have high abrasion resistance. Flame sprayed ceramic coatings containing refractories, such as zirconium oxide, are frequently used for thermal barrier protection of metal subjected to high temperatures. The zirconium oxide usually contains some hafnium oxide and other impurities and can be stabilized with calcium oxide or yttrium oxide.
Typically, one class of ceramic coatings has high thermal resistance and a low wear resistance, while another class of ceramic coatings has a high wear resistance and has a low thermal resistance. The general reason for this relationship is that ceramic coatings which have a high thermal resistance typically are more porous and have a higher void content thereby providing a good thermal barrier but also being less resistant to abrasion. A ceramic coating having a high abrasion resistance has a low void content, thus reducing damage to abrasion. Furthermore, in the prior art, only one specimen is connected to one electrode and the other electrode is connected to the electrolyte tank. Primarily, the power source was single phase AC and DC power.
Although the flame-spray process is widely used, the drawbacks in the process have motivated world wide research into providing a different process for producing materials with improved properties. One area of research is the microarc oxidation of aluminum and its alloys to produce an oxide compound having very high hardness, good wear resistance, and excellent bonding strength with the metallic substrate.
The prior art microarc oxidation processes have been divided into two basic groups. One group is an anodic group on which the substrate is the anode during the process. The other group is the cathodic process in which the substrate is the cathode.
Many techniques have been proposed to form protective ceramic coatings on reactive metals, especially metals such as aluminum and its alloys. One technique was to form such coatings with electrochemical reactions between the metal and an electrolyte solution whereby an oxide coating was formed on the surface of the metal. Such processes involve acid or alkaline based electrolytes. Various techniques were disclosed for establishing a potential whereby the oxide coatings were formed. A class of electrolytic coating methods is micro-arc or microplasmic oxidation. In micro-arc oxidation, the metal is subjected to a high electrical current density while the metal is submerged in an electrolyte bath. Electrical discharges on the surface of the substrate form adherent oxide coatings. Notable among micro-arc oxidation processes is one whereby the potential is pulsed for short periods of time, between 1 to 5 seconds "on" and 1 to 5 seconds "off". While the process produced oxide coatings on the metal substrate it took a considerable period of time, frequently between 5 and 16 hours, to produce a coating between about 50 and 300 microns. The equipment needed to produce such coatings also was complicated and expensive to construct and to operate.
Typically, a substrate of aluminum is subjected to a high electrical current density while submerged in an electrolyte containing a water soluble silicate such as sodium silicate and caustic potash as disclosed in the article entitled, "Special Features of Growth of the Coating in Microarc Oxidation of Aluminum Alloys" by V. N. Kuskov, Yu. N. Kuskov, I. M. Kovenskii, "Physics and Chemistry of Materials Treatment", 1991 25 (5) pp 580-582. The article reports the use of different electrical current densities and the hardness in the coatings over the range tested. This art is also referred to as "microplasmic technology". As used herein, the term "electrical current density" refers to the ratio of the electrical current communicated to a substrate during the process to the exposed surface area of the substrate. The prior art microarc oxidation processes produced good ceramic type coatings but these coatings are limited with respect to the minimum thickness attainable. In addition, the prior art processes did not provide a high degree of control as to the physical qualities of the coatings obtain with respect to selectively achieving good thermal properties or good mechanical properties. Moreover, the prior art has not identified optimum system or the conditions for efficiently operating a system for carrying out the microarc process to obtain superior coatings.
Accordingly, it can be seen that a need exists for a system and an effective process for producing a protective coating or layer on a metal substrate such as aluminum and aluminum alloys.